Abrasive slurries are two-phase systems or dispersions of abrasive particles in liquid(s). Various types of abrasive slurries can be used in machining operations including wire saw slicing, polishing, and planarizing of various materials such as silicon, sapphire, silicon carbide, aluminum, glass, metals, and various ceramics.
Polishing and planarizing processes including Chemical Mechanical Polishing (“CMP”) are surface smoothing and material removal processes that combines chemical and mechanical interactions. In general, the workpiece surface is pressed against a rotating polishing pad, while an abrasive slurry is provided between the surface and the pad. Most of the abrasive slurries used in such cases typically comprise a solid-liquid dispersion system of fine abrasive particles dispersed in an aqueous solution. It is also typical to include in the slurry, in addition to the abrasive, other additives including oxidizing agents, (such as hydrogen peroxide, ferric nitrate, potassium iodate and the like); corrosion inhibitors such as benzotriazole; cleaning agents and surface active agents. In CMP slurries, the abrasive powder provides for mechanical action, while the aqueous solution typically contains reactive chemical agents for chemical action. In general, the abrasive powder abrades the surface to remove protrusions and irregularities in the workpiece surface. The reactive chemical agents provide various functions such as reacting with and/or weakening the material to be removed, aiding in the dissolution of the mechanically removed material by dissolving it into solution, and oxidizing the surface layers to form a protective oxide layer. In many cases, such as slurries involving colloidal silica and ceria particles, the abrasive particles also react with the substrate surfaces to soften the top layer. The polishing pad helps to remove the reacted and abraded materials from the surface. In this way, CMP can be used to flatten and smooth a workpiece to very high levels of local and global planarity.
CMP has been found to be a particularly enabling technology for providing the smooth topographies and uniform thicknesses required in the formation of semiconductor devices. Rapid advances in the semiconductor device industry call for continued increases in wire density and decreases in device size. With these advances, planarization and polishing of the various semiconductor component surfaces becomes more and more critical. Semiconductor devices are typically made by depositing a metal such as copper in spaces between non-conductive structures and then removing the metal layer until the non-conductive structure is exposed and the spaces between remain occupied by the metal. The demands placed on the abrasive are in many ways in conflict. It must remove the metal but preferably not the non-conductive material. It must remove efficiently but not so quickly that the process cannot be terminated when the desired level of removal has been reached.
Many materials requiring planarization and polishing are difficult to polish due to hardness and/or resistance to chemical attack. For example, sapphire (Al2O3), which has been used in forming semiconductor device, is a hard and strong material that transmits ultraviolet, visible, infrared and microwaves, is chemically inert, insoluble in most common industrial solutions, and corrosion resistant, and has low dielectric constant and high thermal conductivity. However, due to sapphire's hardness and resistance to chemical attack, polishing and planarizing sapphire presents many difficulties.
In the past, slurries containing aluminum oxide and silica abrasive particles have been used to polish materials, including sapphire wafers. Colloidal and fumed silicas are desirable for use in abrasive slurries because of their wide availability at a reasonable cost. These silicas further possess colloidal stability in aqueous solutions with a wide variety of chemistries. They also chemically react with the surface, which often enhances material removal. Because silica abrasives are relatively soft, they are capable of polishing a wide variety of surfaces while minimizing defects and scratching. However, the softness of silicas limits their polishing ability and results in low rates of material removal on many types of substrates, such as sapphire, which is hard and resistant to chemical attack. Ceria also provides similar properties for polishing, giving a very good chemical interaction with the substrate surface. However, ceria is often too soft to give adequate removal rate. To compensate for the low material removal rates of silica-containing slurries, harder abrasives are sometimes used. One such material is aluminum oxide. Aluminum oxide has been found to substantially increase removal rates compared with fumed and colloidal silicas in a wide variety of applications. However, it may not give similar chemical reaction which often a critical mechanism for polishing. Also, aluminum oxide cannot directly replace silica particles in applications where silica particles have been used because the surface chemistry of aluminum oxide is very different than that of silica. Specifically, silica has a negative zeta potential over a wide pH range (typically from around pH=2 and higher). Aluminum oxide, on the other hand, has a positive zeta potential over a wide pH range (typically from around pH=9 and lower). As a result, aluminum oxide tends to agglomerate under conditions where silica particles would be stable. Agglomeration decreases the performance of the slurry because it can cause scratching of the workpiece surface as well as uneven and unpredictable polishing rates. In addition, the different surface chemistry of aluminum oxide makes it incompatible with some chemistries used with silica-containing slurries. This can result in surface defects in the substrate if the dispersion is not carefully prepared based on the surface chemistry of aluminum oxide. Further, aluminum oxide is very aggressive such that it is very difficult to avoid “dishing”, which is the formation of a depression in a metal layer lying between adjacent non-conductive material structures. Dishing adversely affects the performance of the semi-conductor and is therefore considered to be very undesirable.
Given these and other deficiencies observed in the art, it would be highly desirable to develop improved abrasive slurry compositions that provide fast removal rate while still minimizing defects and scratching.